DTC U0077: Lin Communication Bus "ECM/Pcm" Off: Notes

DTC detecting condition:

Immediately at fault recognition

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>CLEAR MEMORY MODE>OPERATION , and inspection mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>INSPECTION MODE>PROCEDURE ..
Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram:

Engine Electrical System Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

CHECK DTC.
Read DTC using the Subaru Select Monitor or a general scan tool. Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

Is DTC U0077 displayed? (Current malfunction)

Yes: Go to 2.

No: Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.
CHECK ECM CONNECTOR.
Check the connecting condition of ECM connector.

Is the ECM connector correctly connected?

Yes: Go to 3.

No: Connect the ECM connector correctly.
CHECK HARNESS BETWEEN BATTERY AND BATTERY SENSOR CONNECTOR.
1. Turn the ignition switch to OFF.
2. Disconnect the ground terminal from battery sensor. Refer to NOTE>NOTE>BATTERY .
3. Disconnect the connector from battery sensor.
4. Disconnect the positive terminal of battery.
5. Measure the resistance of the harness between battery positive cable terminal and battery sensor connector.
  Connector & terminal
  
  Positive cable terminal - (B9) No. 1:
Is the resistance less than 1 Ω?

Yes: Go to 4.

No: Repair the open circuit in harness between battery positive cable terminal and battery sensor connector.
CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR.
1. Disconnect the connector from ECM.
2. Measure the resistance of harness between ECM connector and battery sensor connector.
  Connector & terminal
  
  (E158) No. 3 - (B9) No. 2:
Is the resistance less than 1 Ω?

Yes: Repair the poor contact of ECM connector or battery sensor connector.

No: Repair the harness and connector.
NOTE:
In this case, repair the following item:
- Open circuit in harness between ECM connector and battery sensor connector
- Poor contact of coupling connector